Communications transmission links, for example, amplifiers in the receiver or transmitter unit of a mobile telephone, provide non-linear transmission behavior. This nonlinear transmission behavior leads to undesirable amplitude and phase distortions of the signal to be amplified. In order to compensate for these undesired distortion effects, it is already known that an equalizing network, of which the characteristic is ideally designed to be inverse to the non-linear transmission characteristic of the transmission link, can be connected in series to the non-linear transmission link.
The amplitude and phase of the output signal of the transmission link dependent upon the amplitude of the input signal (AM-AM and AM-PM characteristic) are therefore required in order to design the characteristic of the equalizing network. A determination of the amplitude characteristic of the transmission link is obtained from the functional context of the amplitude or respectively power of the signal at the output of the transmission link dependent upon the amplitude or respectively power of the corresponding signal at the input of the transmission link within a defined amplitude or respectively power range of the signal at the input of the transmission link. The phase response of the transmission link once again represents the functional context of the phase change of the signal between the output and input of the transmission link dependent upon the amplitude or respectively power of the signal at the input of the transmission link within a defined amplitude or respectively power range of the signal at the input of the transmission link.
WO 99/05784 A1 describes a method and a device for measuring the amplitude and phase distortion of a high-frequency power amplifier. In this context, the signal at the respective input and output of the high-frequency power amplifier is measured via synchronous demodulators. The ratio of the input to the output amplitude or respectively power is determined in order to present the amplitude characteristic, while, the phase value associated with the respective amplitude or respectively power of the signal at the input is determined in order to present the phase characteristic comprising the in-phase and quadrature components of the output signal. The entire characteristic of the amplitude and phase response is determined by specifying a given signal response at the input of the high-frequency power amplifier by means of a signal generator. The synchronization between the signal at the input and output of the high-frequency power amplifier is implemented via a reference carrier signal between the individual synchronous demodulators.
In calibrating power amplifiers in the receiver and/or transmitter units of mobile telephones, the procedure described in WO 99/05784 A1 of measuring two signals, at the input and at the output of the power amplifier, and the additionally-required synchronization of the two signals is excessively costly in terms of time and functions.